1. Field of the Invention
The present invention relates generally to the production of gypsum board materials, and is more particularly directed to the production and use of high strength gypsum wallboard.
2. Description of Related Technology
One of the most common manners today of constructing walls and barriers includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, often referred to simply as "wallboard" or "drywall." Wallboard can be formulated for interior, exterior, and wet applications. The use of wallboard, as opposed to conventional wet plaster methods, is often desirable because the installation of wallboard is ordinarily less costly than installation of conventional plaster walls.
Generally, wallboard is conventionally produced by enclosing a core of an aqueous slurry of calcined gypsum and other materials between two large sheets of board cover paper. Various types of cover paper are known in the art. After the gypsum slurry has set (i.e., reacted with the water from the aqueous slurry) and dried, the sheet is cut into standard sizes. Methods for the production of gypsum wallboard are described, for example, in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, 1970, Vol. 21, pages 621-24, the disclosure of which is hereby incorporated herein by reference.
Walls and ceilings made with gypsum wallboard panels are conventionally constructed by securing, e.g. with nails or screws, the wallboard panels to structural members, for example vertically and horizontally oriented pieces of steel or wood such as "studs." Because wallboard is typically supplied in standard-sized sheets or panels, when forming a wall from the sheets, there will generally be a number of joints between adjacent sheets. In most wallboard construction, these joints are filled and coated with an adhesive material called a joint compound so that the wall will have a smooth finish similar to that obtained with conventional wet plaster methods.
Gypsum wallboard is typically manufactured commercially by processes that are capable of operation under continuous high speed conditions. The aqueous slurry of calcined gypsum and other ingredients are continuously deposited to form a core between two continuously-supplied moving sheets of cover paper. The calcined gypsum forming the core between the two cover sheets is then allowed to set. The continuously-produced board is cut into panels of a desired length (for example, eight feet) and then passed through a drying kiln in which excess water is removed and the gypsum is brought to a final dry state. After the core has set and is dried, the sandwich becomes a strong, rigid, fire-resistant building material.
A major ingredient of the gypsum wallboard core is calcium sulfate hemihydrate, commonly referred to as "stucco" or "Plaster of Paris." Stucco is commonly manufactured by drying, grinding, and calcining natural gypsum rock. The drying step of stucco manufacture includes passing crude gypsum rock through a rotary kiln to remove any free moisture accumulated in the rock from rain or snow, for example. The dried rock is then passed through a roller mill (a type of pulverizer), wherein the rock is ground to a desired fineness. The dried, ground gypsum can be referred to as "land plaster."
The calcination step is performed by heating the ground gypsum rock, and is described by the following chemical equation: EQU CaSO.sub.4.2H.sub.2 O+heat.fwdarw.CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O.
This chemical equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is conducted in a "calciner," of which there are several types known in the art.
Uncalcined calcium sulfate (the land plaster) is the "stable" form of gypsum. However, calcined gypsum, or stucco, has the valuable property of being chemically reactive with water, and will "set" rather quickly when the two are mixed together. This setting reaction is actually a reversal of the above-described chemical reaction performed during the calcination step. The reaction proceeds according to the following equation: EQU CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O.fwdarw.CaSO.sub.4.2H.sub.2 O+heat.
In this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction is generally dependent upon the type of calciner employed and the type of gypsum rock that is used, and can be controlled within certain limits by the use of additives such as accelerators and retarders.
Because high-weight gypsum wallboard per se is generally not necessary or desirable, various attempts have been made to reduce board weight and density without sacrificing board strength. Heavy gypsum wallboards are more costly and difficult to manufacture than lighter boards. Moreover, in comparison to light boards, heavy boards are more costly and difficult to transport, store, and manually install at job sites.
In order to provide satisfactory strength, commercially-available gypsum wallboard generally requires a density of approximately 1700 pounds (approximately 772 kilograms) per thousand square feet of one-half inch board. It is possible to formulate wallboard having reduced densities through the inclusion of lightweight fillers and foams, for example. Typically, however, where wallboard is formulated to have a density reduced from 1700 pounds per thousand square feet of one-half inch board, the resulting strength is unacceptable for commercial sale.
It would therefore be advantageous to produce high-strength gypsum wallboard having weights and densities reduced from those produced by known methods. Reduced weight and density boards should meet industry standards and have strengths similar to, or greater than, heavier wallboard. Such wallboard also should be able to be manufactured through the use of conventional high-speed manufacturing apparatus and not suffer from other negative side-effects. For example, such high-strength wallboard should be able to set and dry within a reasonable period of time.